1. Field of the Invention
This invention relates to electronic devices for cancelling the effect of an undesired signal from a composite signal.
2. Prior Art
The circuitry associated with a conventional telephone is typically arranged to enable a composite electronic signal representing an additive combination of two acoustic signals to be transmitted to the receiver of the telephone. The composite electronic signal comprises two components which correspond respectively to the two acoustic signals and which may conveniently be termed the sidetone component and the maintone component. The receiver is that part of the telephone which receives the composite electronic signal and converts it into a composite acoustic signal which the telephone user can hear. The telephone user is hereafter generally referred to as the local speaker/listener, and the telephone employed by the local speaker/listener is hereafter generally referred to as the local telephone.
The acoustic signal corresponding to the maintone component is the voice of (or other sounds from) an entity remote to the local telephone. Hereafter, the remote entity is generally referred to as the remote speaker/listener. The acoustic signal of the remote speaker/listener is transformed into an electronic signal and transmitted through a telephone line (or lines) to the local telephone. The electronic signal representing the voice of the remote speaker/listener may be conveniently referred to as the telephone maintone signal.
The acoustic signal corresponding to the sidetone component is the voice (or other sounds from) the local speaker/listener. As the local speaker/listener speaks into (or otherwise has his/her voice conveyed to) the transmitter of the local telephone, the transmitter converts his/her voice into an electronic signal which is normally transmitted to the remote speaker/listener. In addition, the voice of the local speaker/listener is transmitted to the receiver of the local telephone where the local speaker/listener hears his/her own voice. The electronic signal representing the voice of the local speaker/listener carried from the transmitter to the receiver of the local telephone is termed the telephone sidetone signal. Generation of a sidetone signal serves to simulate a person-to-person conversational situation in which an individual hears part of the individual's own voice through the individual's ears.
In the conventional telephone, there is a single electronic path through the receiver for carrying both the maintone and sidetone signals. This arrangement serves to combine the maintone and sidetone signals additively into the composite electronic signal which is received by the receiver.
The sidetone component of the composite signal is typically somewhat reduced in amplitude compared to the amplitude of the actual sidetone signal. The reduction in amplitude is accomplished with a hybrid-coil network connected between the receiver and transmitter of the local telephone. The amount of reduction is controlled by a balancing network connected to the hybrid-coil network. Note that the voice of the local speaker/listener carried from the transmitter to the receiver is normally not completely attenuated in a conventional telephone. That is, the composite electronic signal received at the receiver of the local telephone conventionally always includes a sidetone component representing some actual fraction of the voice of the local speaker/listener.
Hybrid-coil and balancing networks for a conventional telephone are described in Transmission Systems for Communication (3d ed., 1964), Bell Telephone Laboratories Staff, Chap. 3, "Voice Frequency Transmission," pages 49-55. Hybrid-coil and balancing networks for a speakerphone, which is analagous to a conventional telephone as far as the hybrid-coil and balancing networks are concerned, are described by W. F. Clemency, Bell Laboratories Record (June, 1956), "The Speakerphone," vol. 34, no. 6, pages 208-211. Hybrid-coil networks are further discussed in "Engineering Bulletin, Telephone Coupling Transformers" (1974), Triad-Utrad, Litton Systems Inc. These three references are specifically incorporated by reference herein.
In some situations, it is either desirable or necessary to nullify the effect of the sidetone signal. One potential way for eliminating the effect of sidetone signal is to have the local speaker/listener remain silent during the critical period when the effect of the sidetone signal is to be negated. This solution, however, is not feasible if it is also desirable to have the local speaker/listener remain in a continuous conversation with the remote speaker/listener during the critical period. Furthermore, other sounds near the local speaker/listener that are transmitted into the receiver of the local telephone become the sidetone signal.
Another possible solution would be to break into the local telephone and attach appropriate circuitry between the receiver and transmitter to attenuate the sidetone signal completely. This solution, however, is unattractive because it may involve causing physical damage to the local telephone. In addition, breaking into the telephone and attaching the appropriate circuitry to eliminate the sidetone signal would often require more time to implement than is acceptable.
What is needed is a simple, but effective, device that can quickly be attached to the local telephone for cancelling the effect of the telephone sidetone signal without causing any physical damage to the local telephone.